1. Field of the Invention
The present invention generally relates to electronic circuits and, more specifically, to power supply circuits of switched-mode power supply type.
The present invention more specifically applies to switched-mode power supplies intended for display screens, in particular, liquid crystal (LCD) or to active-matrix organic light-emitting diodes (AMOLED) displays.
2. Discussion of the Related Art
FIG. 1 is a simplified block diagram of a display screen, for example, a liquid crystal display of the type to which the present invention more specifically applies.
The display comprises an array 12 of liquid crystal cells controlled by row control circuits 14 (SCAN DRIVERS) and by column control circuits 16 (COLUMN DRIVERS). Circuits 14 and 16 are powered and controlled by circuit 18 (CTRL+POWER) comprising one or several switched-mode power supply circuits.
The display cells require a relatively high power supply voltage (greater than 10 volts, generally on the order of 15 volts) with respect to the voltage that so-called low-voltage components can withstand (less than 10 volts). The cells are generally powered by column drivers 16. These circuits are in a large number (in practice, one per column or one for two columns) and, for integration and miniaturization needs, these circuits are attempted to be formed in a finer technology than that required to withstand the relatively high voltage. For this purpose, column drivers using an intermediary voltage are formed, so that the components of these circuits only have to withstand approximately half of the otherwise required voltage.
The column drivers are then made in the form of cyclic digital-to-analog converters each equipped with an array of switches enabling to select the power supply voltage from among the voltage between the relatively high potential and an intermediary potential and the voltage between the intermediary potential and the reference potential.
The problem of providing these three potentials is then posed.
FIG. 2 is a partial view of a circuit for generating an intermediary voltage, noted HAVDD, between a relatively high voltage VR and ground GND. Voltage VR is provided by a circuit of switched-mode power supply type, powered with a voltage VIN. In applications to liquid crystal displays and to AMOLED-type displays more specifically aimed at by the present invention, voltage VIN is on the order of 12 volts and voltage VR is on the order of from 15 to 18 volts. Power converter 2 is of boost type. It comprises a switch K controlled by pulse-width modulation (block 22, PWM) to switch the current build-up in an inductive element L. For a boost converter, the inductive element and switch K are in series between two terminals 24 and 26 of application of a D.C. voltage. A free wheel diode D, in series with an output capacitor C, connects junction point 28 of inductance L and of switch K to ground 26. Output voltage VR is sampled across capacitive element C. On the power supply voltages side, voltage VIN is, in practice, applied via a switch 21. This switch connects a terminal 23 of application of voltage VIN to terminal 24. Generally, two capacitive smoothing elements 25 and 27 respectively connect terminals 23 and 24 to ground 26. Switch 21 is controlled, for example, by a circuit 29 (CTRL). This circuit may correspond to an output circuit of the display stand-by mode or to any other turn-on switch.
To generate intermediary voltage HAVDD from voltage VR, a follower assembly associated with a resistive dividing bridge formed of two resistors in series between terminal 30 of provision of voltage VR and ground GND is generally used. Their junction point is connected to a non-inverting input of a follower-assembled operational amplifier 34 (having its output terminal looped back on the inverting input). Generally, the intermediary voltage corresponds to half of supply voltage VR. Accordingly, resistors R of same value are used.
On the display side, each column control stage 13 comprises two amplifiers 132 and 134 having their respective outputs intended to be connected via switches, respectively 136 and 138, to a first electrode of a cell 122 of the display, the other electrode of this cell being directly connected to terminal 32 for providing intermediary voltage HAVDD. Switches 136 and 138 are alternately controlled according to the need to bring the first electrode of the cell to a voltage greater than the intermediary voltage or to a lower voltage. For simplification, a single cell 122 has been shown, but it should be noted that a same column amplifier 13 powers all the cells of a same column which are selected by the scan drivers (14, FIG. 1). Amplifiers 132 and 134 receive control signals corresponding to the illumination reference values, as will be better understood hereafter in relation with the discussion of FIG. 3, this operation being known per se.
Elements of protection against electrostatic discharges are provided for the power supply circuit. Typically two diodes D1 and D2 respectively connect terminals 30 and 32 and terminals 32 and 36, their respective anodes being on the side of terminal 32 and on the side of terminal 36, while a third diode D3 connects terminals 30 and 36, its anode being on the side of terminal 36. Although they have been shown on the display side, these elements are generally connected on the column driver side (16, FIG. 1).
A disadvantage of the power supply circuit of FIG. 2 is the dissipation generated by resistors R of the voltage divider and of amplifier 34.